The invention relates to an abradable seal for a gas turbine engine.
In gas turbine engines, some of the aerofoil blades and in particular the turbine blades are conventionally surrounded by a sealing structure, which may comprise an annular seal or a seal segment ring made up of a plurality of arc shaped seal segments. Because the turbine blades expand and contract as their temperatures vary in use and centrifugal loads are imposed upon them, it is normal to provide a small gap between the turbine blade tips and the seal, to allow for this fluctuation.
It is known to provide an abradable seal for sealing between the turbine blade tips and the sealing structure. This enables the tips of the turbine blades to wear away the seal to an optimum size and shape without causing damage to the turbine blade tips. Such abradable seals may consist of an open cell foil honeycomb which is brazed in place and subsequently filled with a suitable abradable material, such as a metallic powder. The foil honeycomb acts as a support for the abradable material and the abradable material and the supporting foil honeycomb is subsequently partially worn away by the rotating turbine blades, thus forming a seal.
Certain problems are associated with the above seals. The seals may suffer from progressive oxidation attack if the foil material has inadequate oxidation resistance. In addition, problems may be experienced with the brazed joints, and the seals may be difficult to cool.
According to the invention, there is provided a sealing element for positioning radially outwardly of at least some of the aerofoil blades of a gas turbine engine, a radially inner surface region of the sealing element including an integrally formed seal structure comprising a plurality of inwardly projecting walls.
The sealing element may comprise or form part of a generally annular housing for surrounding the tips of the blades of the turbine of said engine. The sealing element may comprise a seal segment.
Preferably the walls project substantially radially inwardly.
Preferably radially inner edges of the walls define a substantially arc shaped inner face of the sealing element.
Preferably the seal structure is provided over substantially the whole of a radially inner surface region of the sealing element.
The thickness of the walls may reduce generally towards their radially inner edges.
The walls may be shaped to form a plurality of radially open cells and each cell may be open only at a radially inner side. One or more of the cells may be substantially diamond shaped when viewed in the radial direction. The cells may be all substantially the same size or may be different sizes. The thickness of the walls may increase at their radially inner edges, such that the size of the cells reduces at their open radially inner sides.
According to the invention there is further provided a sealing element as defined in any of the preceding four paragraphs, wherein openings between the walls are at least partially filled with an abradable sealing material. The abradable sealing material may protrude radially inwardly beyond radially inner edges of the walls.
The walls may be abradable. By xe2x80x9cabradablexe2x80x9d it is meant that the material may be worn away by contact with the tips of rotating aerofoil blades, without causing significant damage to the blade tips.
According to the invention there is further provided a seal segment ring for a turbine of a gas turbine engine, the seal segment ring including a plurality of sealing elements as defined in any of the preceding nine paragraphs.
According to the invention there is further provided a gas turbine engine including a turbine comprising a seal segment ring as defined in the preceding paragraph. The turbine may be the high pressure turbine of the gas turbine engine.
According to the invention there is further provided a method of manufacturing a sealing element for positioning radially outwardly of at least some of the aerofoil blades of a gas turbine engine, the method including the step of integrally forming in a radially inner surface region of the seal segment a seal structure comprising a plurality of radially inwardly projecting walls.
The projecting walls may be formed by electro-discharge machining. The method may include the step of machining the walls using an electrode having a generally complementary shape to at least a part of the seal structure.
The method may include the step of manufacturing the electrode by machining the electrode to a generally desired radial profile and subsequently wire cutting grooves into the electrode, the grooves being for subsequently forming the walls of the seal structure.
As an alternative to electro-discharge machining, the method may include the step of integrally casting the walls into the sealing element. Preferably the method includes the further step of electro-chemically machining or etching the seal structure to reduce the thickness of the cast walls.